common/fuzzy: fix some problems with the offline repair strategy

common/fuzzy: fix some problems with the offline repair strategy

While auditing the fuzz tester code, I noticed there were numerous
problems with the offline repair strategy -- the stages of the strategy
are not consistently logged to the kernel log, we don't actually check
that repair -n finds the fuzzed field, and since this is an offline
test, we don't need or want to mount or try to run the online scrubber.
Also, disable prefetch to reduce the chance of an OOM kill.  Rework the
error messages to make reading the golden output easier.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>